This invention deals generally with anchoring means for building columns that are embedded in the earth. Specifically, this invention relates to a method for anchoring vertical wooden posts that are used in the construction of post-frame buildings so the posts resist uplift forces imposed on the structural members without degrading protective materials used on the embedded portion of the post.
Post-frame buildings originated from pole barns and are today used for a wide variety of agricultural, commercial, and industrial purposes since they are, compared to many other types of construction, relatively simple and inexpensive to erect. Conventional post-frame buildings use vertical load bearing wooden posts having their lower ends buried in the earth and their upper ends integrated into the building frame. Suitable footing for the wooden poles is necessary to withstand downward forces from the weight of the building. A typical footing consists of a cylindrical hole approximately 4 feet deep with a concrete base pad in the bottom of the hole on which the post is positioned. At one time, holes were backfilled with compacted earth to maintain the post vertically plumb while the building frame was constructed and also to provide a stable foundation for the completed building. However, increasing building demands on foundations now typically require concrete backfill to provide sufficient foundation strength.
Two problems facing post-frame construction are 1) deterioration of the embedded portion of wooden posts, especially at the interface between the post and the ground surface, and 2) the need to provide a more secure anchorage for the post. The former problem may be addressed through the use of preservative-treated wooden posts to repel insects and decay. However, concrete is known to neutralize many wood-preservative chemicals, promote wood decay, and weaken the structural integrity of wood. An increasingly popular solution in light of costs, environmental risks, and limitations of wood post chemical treatment is the application of protective covers or other water-impermeable media to the embedded portion of the post. U.S. Pat. No. 5,891,583 shows one such protective sleeve. A key consideration in the effectiveness of both preservative treatment and protective sleeves is that care must be taken to avoid breaching the protective barrier as such breaches can lead to premature deterioration of the wood.
Providing sufficient anchorage for the post is becoming increasingly important as post-frame buildings are subjected to safer, more stringent building requirements. One such design consideration is the capability of buildings to resist uplift forces caused by wind loading on the building in which the uplift loads can be on the order of 2,400 pounds per post using 6×6 inch posts. A variety of methods may be used to enhance the foundation capability of embedded posts in response to this requirement. Protrusions may be affixed to the embedded portion of the post exterior surface that extend into the backfill area, thereby increasing the post's ability to withstand pull-out forces. Perhaps the simplest means of adding a protrusion is to bore one or more horizontal holes through the post and to insert a steel bar through the hole. Alternatively, wooden or metallic protrusions can be attached to the external surface of the post using screws, nails, or other fastening hardware. U.S. Pat. No. 6,389,760 discloses a protective sleeve into which the embedded portion of a wooden post is inserted. Protrusions in the form of ridges are molded onto the external surface of the sleeve. These ridges extend into the backfill and provide increased resistance to forces along the post's longitudinal axis.
One shortcoming resulting from use of attached protrusions is the need to breach the wood's external surface or applied protective cover, whether with a hole bored through the post or nails or screws projecting into the post. Preservative treatments are most effective near the external surface of the wooden post. Protection of the interior portions of the post may be less than at the wood surface depending on the effectiveness of the treatment process. Boring a hole thorough the post or using a lag bolt to attach an anchoring device introduces a path for moisture to reach relatively unprotected portions of the wood, which leads to deterioration of the less protected wood. In cases when protective sleeves are employed, any perforation of the protective layer creates a pathway for moisture to reach the wooden post and lead to post deterioration, an especially important consideration since protective sleeves are typically used in lieu of preservative treatment of the wood. Despite the known problems with perforations in protective covers, U.S. Pat. No. 6,389,760 employs one or more lag screws to secure the protective sleeve to the wooden post. The point at which the lag screw penetrates the sleeve is sealed with a neoprene or polyethylene washer to prevent moisture intrusion. In the event that the washer fails, this approach creates a pathway for water intrusion that will ultimately result in premature deterioration of the wooden post.